A hybrid electric vehicle comprises a generator driven by an internal combustion engine and an electric motor driving the vehicle using electrical power generated by the generator. In order to reduce the capacity of a battery of such a vehicle, it is preferred to control the engine output so that power consumed by the electric motor equals the power generated by the generator.
When the vehicle accelerates, a portion of the engine output is used in order to increase the rotation speed of the generator and the engine itself. A time lag results which corresponds to the period from the production of the increase command for the engine output to the time at which the generator reaches a rotation speed which can supply the electrical power required for driving the electrical motor. Such a time lag may be reduced by supplying power from the battery to the generator in order to increase the rotation speed of the engine and the generator. At the same time, power is supplied from the battery to the electric motor to accelerate the vehicle running until the generator reaches a rotation speed at which the required power may be supplied. According to such an operation, however, since power is supplied from the battery to the generator, a reduction in the level of power supply from the battery to the electric motor can not be avoided.
Thus even when battery power is supplied to the generator in order to shorten the time lag, this does not necessarily improve acceleration performance of the vehicle.
JP2001-292501A published by the Japan Patent Office in 2001 discloses a method of limiting the output of the electric motor when power generation by the generator falls below the required power generation amount in order to limit consumption of battery power during vehicle acceleration.